Human scalp-recorded event related potentials (ERPs) have been shown to vary systematically with the information-processing requirements of cognitive tasks. One such task involves the active categorization of stimuli into predefined categories. ERPs elicited by such tasks are sensitive to the effects of stimulus significance (the eliciting stimulus must be relevant), stimulus probability (the ERPs are larger when the probability of presentation is low), and stimulus sequence (ERPs are larger when the eliciting stimulus is preceded by a stimulus of another category). Because of these effects, scalp-recorded ERPs have been studied extensively in an attempt to understand the neurological substrates of cognitive processes. However such efforts have been hampered by the lack of information regarding the neural source(s) for the human scalp-recorded ERPs. Recently Halgren's group at UCLA and Wood's group at the West Haven VA Medical Center both have found task-related ERPs using intracranial recordings from medial temporal lobe that overlap in time with task-related scalp-recorded ERPs during the performance of categorization tasks. Evidence from both groups strongly suggests that task-sensitive ERPs from the medial temporal lobe are local in origin. We propose here to pursue the neural source(s) for these task-sensitive ERPs in an animal model involving two-tone discriminative conditioning of the nictitating membrane response in rabbit. We will record ERPs from multiple skull, cortical and subcortical electrodes (and especially the hippocampus and related structures) during the acquisition, maintenance, reversal and extinction of discriminative NM conditioning. We will evaluate the effects of stimulus significance, stimulus probability, and stimulus sequence upon the simultaneously recorded behavior and ERP measures in order to determine if the rabbit preparation is a good animal model of human task-related ERPs. We eventually want to localize the neurophysiological and neuroanatomical origins of the task-related ERPs. This knowledge may facilitate the use of ERPs as diagnostic tools in cases of suspected cognitive dysfunction.